The present invention relates to sensor modules for the probe of a tactile coordinate measuring machine. More particularly, the invention relates to sensor modules having a stationary module base and a moveable support for holding a stylus, with a deformable connecting element interconnecting the support and the module base.
Coordinate measuring machines are typically used to measure the object shape of a measurement object with high precision. For example, the object shape of a workpiece produced by machine may be checked in the course of quality control. For the measurement process, the probe of the coordinate measuring machine is moved toward the measurement object by means of an appropriate movement mechanism until the stylus touches a desired measurement point on the measurement object. The spatial coordinates of the measurement point can then be determined from the position of the probe and from the relative position of the stylus with respect to the probe.
Usually, the stylus is movably suspended on the probe via spring mechanisms and/or plain or roller bearings. Plunger coils or Hall sensors, for example, are used in order to detect the deflections of the stylus relative to the probe. Such “conventional” probe kinematics are several centimeters in size. By contrast, DE 101 08 774 A1 describes a sensor module for a miniaturized probe in an exemplary embodiment. The sensor module comprises a silicon monocrystal, and the structure of the module was fabricated by etching from a solid silicon body. The edge length of the entire sensor module is only 6 mm, while the profile thickness of the module base is 0.5 mm. The deformable connecting element via which the likewise miniaturized stylus is connected to the module base comprises silicon webs with a thickness of approximately 30 μm that have been left standing upon complete etching of the solid body. When the stylus is deflected, the webs are twisted, and this can be detected by means of strain sensors. Owing to the miniaturization and the fragile design, this sensor module is the basis of a new generation of probes for tactile coordinate measuring machines by means of which very accurate measurements on microstructures are rendered possible because of the small dimensions and deflections.
The fundamental concept of such a sensor module for a tactile probe is disclosed in an article by Kleine-Besten et al. entitled “Miniaturisierter 3D-Tastsensor für die Metrologie an Mikrostrukturen” [“Miniaturized 3D probe sensor for metrology of microstructures”], which appeared in the German journal “tm-Technisches Messen” [“tm-Technical measurement”], issue December 1999, pages 490-495. This article describes investigation results on a miniaturized sensor module of this kind, wherein, in contrast to the exemplary embodiment from DE 101 08 774 A1, the support for holding the stylus (the so-called “boss”) is held on the module base by a solid membrane. The use of individual webs for holding the boss, as described in DE 101 08 774 A1, is mentioned in the article as a prospect for matching the bending stiffnesses of the module in the three spatial directions x, y, z with respect to one another. This is because the investigation of the sensor module with a solid membrane has shown that the bending stiffnesses when the stylus is deflected in a plane parallel to the module base (x-direction or y-direction) are considerably less than when it is deflected at right angles to the module base (z-direction).
Securing the boss or stylus to the module base by means of webs already leads to an improved matching of the bending stiffnesses. However, there are still considerable difficulties when measurement processes in which the probe is guided in continuous contact with the measurement object are to be carried out (what is called scanning). For such complex measurement processes, it is still problematic to exactly determine the deflection in the z-direction, despite the matching of the bending stiffnesses already achieved.
A probe having a miniaturized sensor module is also disclosed in DE 103 14 304 A1 and WO 2004/068068 having the same priority date. However, these publications relate to the mechanical fastening of the fragile sensor module to a probe. The problem of different bending stiffnesses is not a topic here.
In an older patent application (not a prior publication) of the present applicant (PCT/EP2005/005622), a sensor module of the type mentioned at the beginning is described, wherein the webs between the module base and the stylus support (boss) are designed with a cross section of varying material thickness. This implementation leads to an improved matching of the bending stiffnesses such that scanning measurements can be carried out relatively well. However, the bending stiffnesses in the x-/y-direction and z-direction are still not identical with this solution. Moreover, the production of this novel sensor module requires very fine etching structures that must be executed with high accuracy, and this constitutes a challenge of production engineering.